Vehicle vision system with enhanced camera brightness control

ABSTRACT

A vision system for a vehicle includes a camera disposed at a vehicle and having a field of view exterior of the vehicle. A brightness control, responsive to processing of a frame of image data captured by the camera, interpolates towards an expected brightness for a next frame of captured image data by calculating a set of at least three brightness values using three different control coefficients derived from a previous frame of image data captured by said camera. The brightness control interpolates toward the expected brightness of the next frame of captured image data using the current expected brightness value and two of the three brightness values derived from the three control coefficients derived from the previous frame of captured image data.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the filing benefits of U.S. provisionalapplication Ser. No. 62/369,775, filed Aug. 2, 2016, which is herebyincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to a vehicle vision system for avehicle and, more particularly, to a vehicle vision system that utilizesone or more cameras at a vehicle.

BACKGROUND OF THE INVENTION

Use of imaging sensors in vehicle imaging systems is common and known.Examples of such known systems are described in U.S. Pat. Nos.5,949,331; 5,670,935 and/or 5,550,677, which are hereby incorporatedherein by reference in their entireties.

SUMMARY OF THE INVENTION

The present invention provides a driver assistance system or visionsystem or imaging system for a vehicle that utilizes one or more cameras(preferably one or more CMOS cameras) to capture image datarepresentative of images exterior of the vehicle, and provides abrightness control that controls brightness parameters of the camera orimage sensor. An image processor is operable to process image datacaptured by the camera. The brightness control, responsive to brightnessvalues of a previous frame of image data, interpolates towards anexpected brightness by calculating a set of at least three brightnessvalues from the previous frame of image data with three differentcontrol coefficients, from which two are chosen in between set points(of expected or desired brightnesses) for further processing. The systemrepeats this process over multiple frames of captured image data,adjusting the coefficients based the previous frame and on an errorbetween the previous frame brightness and the expected brightness, toadjust the camera brightness until it is within a threshold level fromthe expected brightness.

These and other objects, advantages, purposes and features of thepresent invention will become apparent upon review of the followingspecification in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a vehicle with a vision system thatincorporates cameras in accordance with the present invention;

FIGS. 2 and 3 are schematics showing known brightness control;

FIG. 4 shows a brightness transfer function, where the B_(i) axis is theactual pixel brightness and the B_(o) axis is the output brightness,shown with four exemplary global coefficient's curves, with the controldependency being linear which would be ideal;

FIG. 5 shows a brightness transfer function, where the B_(i) axis is theactual pixel brightness and the B_(o) axis is the output brightness,shown with four exemplary global coefficient's curves, with the controldependency being non-linear;

FIG. 6 is a schematic showing a brightness control process in accordancewith the present invention;

FIG. 7 is a schematic in accordance with the present invention showinghow three brightness values get determined from three coefficients (fromthe coefficient of a former frame i−1 plus/minus a deviation of afunction of the difference e (of the actual brightness and the expectedbrightness, or simpler a constant (fixed delta)) in a first step (1) and(backward-) interpolating a new coefficient of the current frame i fromthe ratio of the expected brightness to the two closest determinedbrightness results in a second step (2);

FIG. 8 is a flow chart of the coefficient determination of the currentframe using the steps shown in FIG. 7 in accordance with the presentinvention;

FIG. 9 is a timing diagram of the coefficient determination of thecurrent frame using the steps shown in FIG. 7 in accordance with thepresent invention;

FIG. 10 is a table showing resource utilization;

FIG. 11 shows trace properties of a brightness control algorithmaccording the invention, where the actual brightness steps areproportional to the Δ value (ƒ(e)=Δ);

FIG. 12 shows trace properties of a brightness control algorithmaccording the invention, where the function of the difference e isƒ(e)=k*e (proportional, continuous and linear) such as with being ascaling factor coefficient;

FIG. 13 shows trace properties of a brightness control algorithmaccording the invention, where the function of the difference e isƒ(e)=k*e with p being chosen high so that the system adapts quickly butswings more than one time past a brightness set point jump stimuli;

FIG. 14 shows a graph of the correction (coefficient) output independency of the calculated correction (coefficient), with the lowerend truncated to zero (dead zone); and

FIG. 15 shows a graph of the correction (coefficient) output independency of the calculated correction (coefficient), with the lowerend truncated to zero (dead zone) and the top end limited to a maximumvalue.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A vehicle vision system and/or driver assist system and/or objectdetection system and/or alert system operates to capture images exteriorof the vehicle and may process the captured image data to display imagesand to detect objects at or near the vehicle and in the predicted pathof the vehicle, such as to assist a driver of the vehicle in maneuveringthe vehicle in a rearward direction. The vision system includes an imageprocessor or image processing system that is operable to receive imagedata from one or more cameras and provide an output to a display devicefor displaying images representative of the captured image data.Optionally, the vision system may provide display, such as a rearviewdisplay or a top down or bird's eye or surround view display or thelike.

Referring now to the drawings and the illustrative embodiments depictedtherein, a vehicle 10 includes an imaging system or vision system 12that includes at least one exterior viewing imaging sensor or camera,such as a rearward facing imaging sensor or camera 14 a (and the systemmay optionally include multiple exterior viewing imaging sensors orcameras, such as a forward viewing camera 14 b at the front (or at thewindshield) of the vehicle, and a sideward/rearward viewing camera 14 c,14 d at respective sides of the vehicle), which captures images exteriorof the vehicle, with the camera having a lens for focusing images at oronto an imaging array or imaging plane or imager of the camera (FIG. 1).Optionally, a forward viewing camera may be disposed at the windshieldof the vehicle and view through the windshield and forward of thevehicle, such as for a machine vision system (such as for traffic signrecognition, headlamp control, pedestrian detection, collisionavoidance, lane marker detection and/or the like). The vision system 12includes a control or electronic control unit (ECU) or processor 18 thatis operable to process image data captured by the camera or cameras andmay detect objects or the like and/or provide displayed images at adisplay device 16 for viewing by the driver of the vehicle (althoughshown in FIG. 1 as being part of or incorporated in or at an interiorrearview mirror assembly 20 of the vehicle, the control and/or thedisplay device may be disposed elsewhere at or in the vehicle). The datatransfer or signal communication from the camera to the ECU may compriseany suitable data or communication link, such as a vehicle network busor the like of the equipped vehicle.

Conventional vehicle camera brightness controls typically employ a loopcontrol. The brightness output is the feedback for the deviation fromthe desired brightness set point. The brightness can be calculated underuse of a cumulative histogram, where, for example, the 80 percent binmay be the indicator of the brightness. To decrease or increase thebrightness, there may be a coefficient which is a gain factor to thepixels before running the cumulative histogram. The coefficient can be acontrolling parameter in a non-linear function, for example, ahyperbolic tangent function. By that the coefficient can be used ascontrol for the brightness. Since the histogram's profile varies, thehistogram cannot be expressed as a continuous mathematical function, andthe single pixel's brightness is not linearly linked to the cumulativebrightness result, the optimal coefficient cannot be calculated apriori, but must be estimated or assumed as best as possible in advanceof each frame. A typical way is to predict the best coefficient for thenext frame from the knowledge of the previous frames, so by loop (orfeedback) control, is illustrated in FIG. 2.

The brightness (loop) control is typically a PID-, PD-, DI-, or PI-, orjust a P-type. The parameters of the proportional, the integral and thedifferential control part are typically fixedly set. In case the overalldampening is below one, the system adapts to the (user desired) setpoint faster, but overshoots more and oscillates longer, and in the casethe overall system's dampening above one, there is no or less overswing, but the desired set point or image brightness changes are reachedlater. Over swing, oscillation and slow brightness adaption are notdesired in brightness control. The parameters ofproportional-integral-derivative (PID) controls are typically set(optimized) in a way that the system dampening is close to 1, but sincethe control system is not continuous, there is no optimal PID setting. APID controlled brightness control setting may be a compromise betweenthe lackings of being slow or over swinging. A brightness always varies(needs to be adapted to) when the light condition in the capture imagechanges and the dynamic range of the camera, display and/or processingsystem is not wide enough.

Brightness Control in Practice:

Brightness control approaches mainly fall into 2 categories:

-   -   (1) Within the image sensor. The brightness parameters, such as        target brightness, integration time and/or global gain, are sent        via I2C from the control unit to the image sensor. Then the        image sensor updates those parameters for next frames and then        the brightness has new values. A benefit of this approach is        that the image sensor takes over the brightness adaption        natively. However, I2C communication is slow and the reaction        for the parameter change is slow.    -   (2) After the image sensor. The image sensor has the constant        parameters related to brightness. The control unit processes the        raw image and outputs a new image with expected brightness. A        benefit of this approach is that the image sensor keeps in the        same state and the user can elaborate specific brightness        control strategy. However, additional effort may be needed for        the control unit design.

For the second approach, PID control is popular, such as shown in FIG.3. PID is also available for the first approach.

Instead of using an comparably slow PID control for adapting the outputbrightness to a varying input brightness, the camera brightness controlof the present invention interpolates towards the set point (expectedbrightness B_(exp)) by calculating a set of three (or five or optionallymore) brightnesses from three (or five or optionally more) differentcontrol coefficients, from which two are chosen in between the setpoints, for further processing (see FIG. 6). This may be done in acontrol of category two (after the image sensor).

For example, for every current frame of image or image data, there aretwo hidden frames running parallel (instantaneously) with it. Thesethree frames of image data have their own brightness coefficients andbrightness result. Due to the nonlinearity between the coefficient andthe brightness (see FIGS. 4 and 5 in comparison) and the discontinuousbehavior, this solution gives a more stable and faster brightnesstracing without overshooting (see FIGS. 11 and 12) compared to a PIDcontrolled brightness control. Switching between the brightnesscoefficients according the present invention essentially equates to aswitching between I (Integral) coefficients. Making the coefficient afunction of the error makes the I-component adaptive to the deviation.The adaption may be proportional, exponential or any other function ormay be a discontinuous relationship. By that the control of the presentinvention is an I-type with optional deviation (or error) adaptiveI-parameter (or coefficient). The varying I parameter determines theslope at which the actual brightness value conveys towards the expectedbrightness value (set point).

Note that in FIG. 7, ƒ(e) is a function of the current difference (orerror) e (as in FIG. 3), which means that, for larger brightnessvariation between frames, the trying scope ƒ(e) will also be larger, andvice versa. The function ƒ(e) can be linear, non-linear (such as, forexample, an exponential curve, a saturation curve, hyperbolic, or apolygon) or discontinuous. In practice, it may be sufficient to set thisfunction as a constant to simplify the implementation, for example,ƒ(e)=Δ (an according slope diagram is shown in FIG. 11). FIG. 12 showsan example of a trace property of an algorithm, where the function ofthe difference “e” (for error) is proportional (and continuous)(ƒ(e)=k*e; with k as a constant).

The present invention provides enhanced fast online brightness controlthrough coefficient interpolation (see FIGS. 7 and 8):

${\frac{\partial{e}}{\partial C_{i}} = {{\frac{\partial}{\partial C_{i}}\left( {{{B\left( C_{i} \right)} - B_{{ex}\; p}}} \right)}->0}},{C^{-} \leq C_{i} \leq C^{+}},$

under use of the minimal brightness deviation e:min(e)=min(|B(C _(i))−B _(exp)|),B ⁻ ≤B≤B ⁺.

Image brightness (B) is controlled by a coefficient (C), in the solutionof the present invention (see FIG. 9), by a knee-point parameter for theglobal brightness adjustment. The coefficient has good correlation (forexample, tan h) to the image brightness and does not impact the originalimage sensor behavior. This means that the coefficient is calculated ina shorter time period and can be used at once for the next image.

Conventionally, a brightness control requirement may be fulfilled by PIDcontrol. But for real-time purposes, the PID introduces complexity ofparameter selection. The PID control has at least 3 parameters to adjustand it is difficult to find an optimal combination for all lightingconditions.

The system or control of the present invention takes advantage offield-programmable gate array (FPGA) parallel processing on the originalimage and has 3 versions of brightness values out of 3 differentprevious coefficients simultaneously. The 3 coefficients are chosen asthe previous coefficient (Coef_(i-1)), a larger coefficient (+) and asmaller coefficient (−), so that there are correspondingly 3 brightnessvalue results (B, B⁺ and B⁻). The next coefficient (Coef_(i)) iscalculated by interpolation among these 3 coefficients with respect tosmallest error to the 3 brightness values. Since the FPGA cannot holdthe current frame in memory, the new coefficient may be used for thenext frame's brightness determination. Optionally, with very fastsystems or systems with long frame pause time and a frame memory, thebrightness of the current frame may be calculated out of the new foundcoefficient instantaneously.

The interpolation works as in the formula below, where B denotesexpected brightness and ε the dead-zone. In the strip of ∥ε∥ thebrightness Coef_(i) will not change.

${Coef}_{i} = \left\{ \begin{matrix}{{{Coef}_{i - 1} + {f(e)}},} & {{{if}\mspace{14mu}\overset{\overset{\_}{\_}}{B}} \geq B^{+}} \\{{{Coef}_{i - 1} + {\frac{\overset{\overset{\_}{\_}}{B}\mspace{14mu} B^{0}}{B^{+} - B^{0}}{f(e)}}},} & {{{{if}\mspace{14mu} B^{0}} + ɛ} < \overset{\overset{\_}{\_}}{B} < B^{+}} \\{{{Coef}_{i - 1} - {\frac{B^{0} - \overset{\overset{\_}{\_}}{B}}{B^{0} - B^{-}}{f(e)}}},} & {{{if}\mspace{14mu} B^{-}} < \overset{\overset{\_}{\_}}{B} < {B^{0} - ɛ}} \\{{{Coef}_{i - 1}{f(e)}},} & {{{if}\mspace{14mu}\overset{\overset{\_}{\_}}{B}} \leq B^{-}}\end{matrix} \right.$

A dead-zone ∥ε∥ is needed to have a stable video (nun pumping orflickering). The dead zone ∥ε∥ may be a tolerance band above and belowthe error e (see FIGS. 11-14).

In case the calculated coefficient correction (as a function of thecurrent difference or error e) is within the dead zone boundary, thecorrection will be ignored so the new brightness coefficient will beidentical to the former, see FIG. 14. Optionally, inside the dead-zone,an accumulative deviation may be used for a fine tuning.

FIG. 11 shows a trace property of this algorithm, where the actualbrightness steps are proportional to the Δ value (ƒ(e)=Δ). On abrightness jump stimuli the control is ramping towards the set point inidentical step width. A litte delta remains when the actual value isclose to the set point, hitting the dead zone.

When an algorithm according the invention uses a function of thedifference e which is linear such as being ƒ(e)=k*e, with k being ascaling factor coefficient or non-linear such as being ƒ(e)=2^(k)*^(e),with k being a scaling factor coefficient in the exponent theconvergence to the actual brightness may be significantly faster.

FIG. 12 shows a trace property of an algorithm, where the function ofthe difference e is proportional (and continuous), such as beingƒ(e)=k*e, with k being a scaling factor coefficient. On a brightnessjump stimuli the control is ramping towards the set point in largersteps at the beginning and smaller steps at the end. A litte deltaremains when the actual value is close to the set point, hitting thedead zone. No overshooting appears. The factors may be chosen properly.Attention should be paid with this variable correction algorithm thatthe acceleration factor is not too large to get a stable brightnesstracing (swinging) (see FIG. 13). For the fastest control setting, thefactor may always be aligned with ∥ε∥ so that the ∥ε∥ band may bereached past one over swing in maximum.

Optionally, the coefficient may be limited to a maximal value, shown inthe graph of FIG. 15. A benefit of this solution is a better and fasterconvergence together with a high stability. This approach avoidsover-tuning even under quickly alternating lighting conditions. Thecomputational operations are limited to multiplications and summationseasy to implement in FPGAs.

With extreme lighting conditions (too bright and too dark), the controlmodule can change the global gain or integration time of the imagesensor in very low possibility.

This brightness control can also target on a sub region, such as aregion of interest (ROI).

Optionally, the system of the present invention may incorporate imagenoise filtering methods and algorithms such as by utilizing aspects ofthe vision systems described in U.S. Publication No. US-2015-0042806,which is hereby incorporated herein by reference in its entirety.

Optionally, the system of the present invention may incorporate enhancedlow light capability methods and algorithms such as by utilizing aspectsof the vision systems described in U.S. Publication No. US-2014-0354811,which is hereby incorporated herein by reference in its entirety.

Optionally, the system of the present invention may incorporate shadingcorrection methods and algorithms such as by utilizing aspects of thevision systems described in U.S. provisional application Ser. No.62/448,091, filed Jan. 19, 2017, which is hereby incorporated herein byreference in its entirety.

Optionally, the system of the present invention may incorporate testmethods and devices such as by utilizing aspects of the vision systemsdescribed in U.S. provisional application Ser. No. 62/486,072, filedApr. 17, 2017, which is hereby incorporated herein by reference in itsentirety.

The camera or sensor may comprise any suitable camera or sensor.Optionally, the camera may comprise a “smart camera” that includes theimaging sensor array and associated circuitry and image processingcircuitry and electrical connectors and the like as part of a cameramodule, such as by utilizing aspects of the vision systems described inInternational Publication Nos. WO 2013/081984 and/or WO 2013/081985,which are hereby incorporated herein by reference in their entireties.

The system includes an image processor operable to process image datacaptured by the camera or cameras, such as for detecting objects orother vehicles or pedestrians or the like in the field of view of one ormore of the cameras. For example, the image processor may comprise animage processing chip selected from the EyeQ family of image processingchips available from Mobileye Vision Technologies Ltd. of Jerusalem,Israel, and may include object detection software (such as the typesdescribed in U.S. Pat. Nos. 7,855,755; 7,720,580 and/or 7,038,577, whichare hereby incorporated herein by reference in their entireties), andmay analyze image data to detect vehicles and/or other objects.Responsive to such image processing, and when an object or other vehicleis detected, the system may generate an alert to the driver of thevehicle and/or may generate an overlay at the displayed image tohighlight or enhance display of the detected object or vehicle, in orderto enhance the driver's awareness of the detected object or vehicle orhazardous condition during a driving maneuver of the equipped vehicle.

The vehicle may include any type of sensor or sensors, such as imagingsensors or radar sensors or lidar sensors or ladar sensors or ultrasonicsensors or the like. The imaging sensor or camera may capture image datafor image processing and may comprise any suitable camera or sensingdevice, such as, for example, a two dimensional array of a plurality ofphotosensor elements arranged in at least 640 columns and 480 rows (atleast a 640×480 imaging array, such as a megapixel imaging array or thelike), with a respective lens focusing images onto respective portionsof the array. The photosensor array may comprise a plurality ofphotosensor elements arranged in a photosensor array having rows andcolumns. Preferably, the imaging array has at least 300,000 photosensorelements or pixels, more preferably at least 500,000 photosensorelements or pixels and more preferably at least 1 million photosensorelements or pixels. The imaging array may capture color image data, suchas via spectral filtering at the array, such as via an RGB (red, greenand blue) filter or via a red/red complement filter or such as via anRCC (red, clear, clear) filter or the like. The logic and controlcircuit of the imaging sensor may function in any known manner, and theimage processing and algorithmic processing may comprise any suitablemeans for processing the images and/or image data.

For example, the vision system and/or processing and/or camera and/orcircuitry may utilize aspects described in U.S. Pat. Nos. 9,233,641;9,146,898; 9,174,574; 9,090,234; 9,077,098; 8,818,042; 8,886,401;9,077,962; 9,068,390; 9,140,789; 9,092,986; 9,205,776; 8,917,169;8,694,224; 7,005,974; 5,760,962; 5,877,897; 5,796,094; 5,949,331;6,222,447; 6,302,545; 6,396,397; 6,498,620; 6,523,964; 6,611,202;6,201,642; 6,690,268; 6,717,610; 6,757,109; 6,802,617; 6,806,452;6,822,563; 6,891,563; 6,946,978; 7,859,565; 5,550,677; 5,670,935;6,636,258; 7,145,519; 7,161,616; 7,230,640; 7,248,283; 7,295,229;7,301,466; 7,592,928; 7,881,496; 7,720,580; 7,038,577; 6,882,287;5,929,786 and/or 5,786,772, and/or U.S. Publication Nos.US-2014-0340510; US-2014-0313339; US-2014-0347486; US-2014-0320658;US-2014-0336876; US-2014-0307095; US-2014-0327774; US-2014-0327772;US-2014-0320636; US-2014-0293057; US-2014-0309884; US-2014-0226012;US-2014-0293042; US-2014-0218535; US-2014-0218535; US-2014-0247354;US-2014-0247355; US-2014-0247352; US-2014-0232869; US-2014-0211009;US-2014-0160276; US-2014-0168437; US-2014-0168415; US-2014-0160291;US-2014-0152825; US-2014-0139676; US-2014-0138140; US-2014-0104426;US-2014-0098229; US-2014-0085472; US-2014-0067206; US-2014-0049646;US-2014-0052340; US-2014-0025240; US-2014-0028852; US-2014-005907;US-2013-0314503; US-2013-0298866; US-2013-0222593; US-2013-0300869;US-2013-0278769; US-2013-0258077; US-2013-0258077; US-2013-0242099;US-2013-0215271; US-2013-0141578 and/or US-2013-0002873, which are allhereby incorporated herein by reference in their entireties. The systemmay communicate with other communication systems via any suitable means,such as by utilizing aspects of the systems described in InternationalPublication Nos. WO 2010/144900; WO 2013/043661 and/or WO 2013/081985,and/or U.S. Pat. No. 9,126,525, which are hereby incorporated herein byreference in their entireties.

Optionally, the vision system may include a display for displayingimages captured by one or more of the imaging sensors for viewing by thedriver of the vehicle while the driver is normally operating thevehicle. Optionally, for example, the vision system may include a videodisplay device, such as by utilizing aspects of the video displaysystems described in U.S. Pat. Nos. 5,530,240; 6,329,925; 7,855,755;7,626,749; 7,581,859; 7,446,650; 7,338,177; 7,274,501; 7,255,451;7,195,381; 7,184,190; 5,668,663; 5,724,187; 6,690,268; 7,370,983;7,329,013; 7,308,341; 7,289,037; 7,249,860; 7,004,593; 4,546,551;5,699,044; 4,953,305; 5,576,687; 5,632,092; 5,677,851; 5,708,410;5,737,226; 5,802,727; 5,878,370; 6,087,953; 6,173,508; 6,222,460;6,513,252 and/or 6,642,851, and/or U.S. Publication Nos.US-2012-0162427; US-2006-0050018 and/or US-2006-0061008, which are allhereby incorporated herein by reference in their entireties. Optionally,the vision system (utilizing the forward facing camera and a rearwardfacing camera and other cameras disposed at the vehicle with exteriorfields of view) may be part of or may provide a display of a top-downview or birds-eye view system of the vehicle or a surround view at thevehicle, such as by utilizing aspects of the vision systems described inInternational Publication Nos. WO 2010/099416; WO 2011/028686; WO2012/075250; WO 2013/019795; WO 2012/075250; WO 2012/145822; WO2013/081985; WO 2013/086249 and/or WO 2013/109869, and/or U.S.Publication No. US-2012-0162427, which are hereby incorporated herein byreference in their entireties.

Changes and modifications in the specifically described embodiments canbe carried out without departing from the principles of the invention,which is intended to be limited only by the scope of the appendedclaims, as interpreted according to the principles of patent lawincluding the doctrine of equivalents.

The invention claimed is:
 1. A vision system for a vehicle, said visionsystem comprising: a camera disposed at a vehicle and having a field ofview exterior of the vehicle; a processor operable to process image datacaptured by said camera; a brightness control; and wherein saidbrightness control, responsive to image processing of a current frame ofimage data captured by said camera, interpolates towards an expectedbrightness for a next frame of captured image data by calculating a setof at least three brightness values using three different controlcoefficients derived from a previous frame of image data captured bysaid camera, and wherein said brightness control interpolates toward theexpected brightness of the next frame of captured image data using thecurrent expected brightness value and two of the three brightness valuesderived from the three control coefficients derived from the previousframe of captured image data.
 2. The vision system of claim 1, whereinsaid vision system processes each frame of captured image data as threeframes, each having its own control coefficient and brightness value. 3.The vision system of claim 1, wherein the three control coefficients areselected as a previous control coefficient, a larger control coefficientand a smaller control coefficient, so that there are correspondinglythree brightness values.
 4. The vision system of claim 3, wherein a nextcontrol coefficient is calculated by interpolation among the threeselected control coefficients with respect to the three brightnessvalues.
 5. The vision system of claim 3, wherein the larger and smallercontrol coefficients are at least in part derived from an error valuethat is determined as the difference between an expected brightness andbrightness from a previous frame of captured image data.
 6. The visionsystem of claim 5, wherein the three control coefficients comprise abase control coefficient, an upper control coefficient that comprisesthe base control coefficient plus the error value, and a lower controlcoefficient that comprises the base control coefficient minus the errorvalue.
 7. The vision system of claim 5, wherein said brightness controlselects one of the control coefficients based on the expected brightnesslevel compared to the three brightness levels, and wherein the selectedcontrol coefficient is applied to the brightnesses determined from thenext frame of captured image data.
 8. The vision system of claim 4,wherein the larger and smaller control coefficients are at least in partderived from a function of an error value that is determined as thedifference between an expected brightness and brightness from a previousframe of captured image data.
 9. The vision system of claim 8, whereinthe function comprises a non-linear function.
 10. The vision system ofclaim 8, wherein the function comprises a linear function.
 11. Thevision system of claim 1, wherein said brightness control interpolatestowards an expected brightness when a difference between the currentbrightness and the expected brightness is greater than a thresholdlevel.
 12. A method for adjusting a brightness setting of a camera for avehicular vision system, said method comprising: (a) providing a cameraconfigured to be disposed at a vehicle so as to have a field of viewexterior of the vehicle; (b) providing a processor operable to processimage data captured by the camera; (c) providing a brightness controloperable to control the brightness setting of the camera; (d) capturinga current frame of image data by the camera; (e) processing the currentframe of image data to determine a difference between an actualbrightness for the current frame of image data and an expectedbrightness for the current frame of image data; (f) determining threecoefficients for the current frame of image data derived from thedifference between the actual brightness for the current frame of imagedata and the expected brightness, wherein at least some of the threecoefficients are derived from a previous frame coefficient and thedifference between the actual brightness for the current frame of imagedata and the expected brightness; (g) applying the three coefficientsfor the current frame of image data to the current frame of image datato determine three brightness values for the current frame of imagedata, each of the three brightness values derived from a respective oneof the three coefficients; (h) interpolating the expected brightness andthe three brightness values to determine a frame coefficient that willbe used as the previous frame coefficient during processing of asubsequent frame of image data; and (i) repeating steps (d) to (h) untilthe difference between the actual brightness and the expected brightnessfor the then current frame of image data is within a threshold range.13. The method of claim 12, wherein the three coefficients for thecurrent frame of image data are selected as a mid-coefficient, a largercoefficient and a smaller coefficient, so that there are correspondinglythree brightness values.
 14. The method of claim 13, wherein the largerand smaller coefficients are at least in part derived from thedifference between the expected brightness and actual brightness fromthe previous frame of captured image data.
 15. The method of claim 14,wherein the larger coefficient comprises the mid-coefficient plus thedifference value, and the smaller coefficient comprises themid-coefficient minus the difference value.
 16. The method of claim 12,wherein said camera continues to capture image data and said brightnesscontrol continues to interpolate towards an expected brightness as longas the difference between the brightness of the then current frame ofimage data and the expected brightness is greater than a thresholdlevel.
 17. The method of claim 12, wherein determining threecoefficients derived from the difference between the actual brightnessfor the current frame of image data and the expected brightnesscomprises determining three coefficients derived from a function of thedifference.
 18. The method of claim 17, wherein the function comprises anon-linear function.
 19. A vision system for adjusting a brightnesssetting of a camera for a vehicular vision system, said vision systemcomprising: a camera configured to be disposed at a vehicle so as tohave a field of view exterior of the vehicle; a processor operable toprocess frames of image data captured by the camera; a brightnesscontrol operable to control the brightness setting of the camera;wherein, responsive to processing a current frame of image data capturedby the camera, said brightness control determines a difference betweenan actual brightness for the current frame of image data and an expectedbrightness; wherein said brightness control determines threecoefficients based at least in part on the difference between the actualbrightness for the current frame of image data and the expectedbrightness for the current frame of image data captured by the camera;wherein at least some of the three coefficients are derived from apreviously determined coefficient determined via processing a previousframe of captured image data; wherein said brightness control appliesthe three coefficients for the current frame of image data to thecurrent frame of image data to determine three brightness values for thecurrent frame of image data, each of the three brightness values derivedfrom a respective one of the three coefficients; wherein said brightnesscontrol interpolates the expected brightness and the three brightnessvalues to determine a current frame coefficient that will be used as aprevious frame of image data coefficient during processing of asubsequent frame of image data; and wherein said vision system continuesto process frames of captured image data until the difference betweenthe actual brightness and the expected brightness for a current capturedframe of image data is within a threshold range.
 20. The vision systemof claim 19, wherein the three coefficients are derived from a functionof the difference between the actual brightness for the current frame ofimage data and the expected brightness, and wherein the functioncomprises a non-linear function.